1. Field of the Invention
This invention relates to signal processing and, in particular, to structures and processes for processing signals using an analog/multi-level memory.
2. Description of Related Art
FIG. 1 is a block diagram of a typical digital image processing system 100, such as a digital still camera. An image sensor 110, for example, a charge-coupled device (CCD) or CMOS sensor, first converts the image to electrical signals, such as voltages. An analog-to-digital (A/D) converter 120 then samples the analog signal and converts the signal into digital data. Some advantages of digitized information include processing flexibility and higher signal to noise ratio during transmission to reduce error rates. However, digital data can require very large memory storage capacities and transmission bandwidths.
For example, typical VGA images have a spatial resolution of approximately 307K pixels/frame (i.e., 480 rows with 640 samples or pixels per row). In a digital image, the image is captured, processed, stored, and transmitted as an array of numerical values. The image is divided up into squares in a grid, with each square in the grid referred to as a pixel or sample. The intensity of the image at each pixel is translated into a numerical value which is stored in the array. If the pixel has one of the primary colors (i.e., R, G, B) to be sampled with 8 bits resolution, then there are approximately 2.5 Mbits of data per frame (307K*8), which requires approximately 307 Kbytes of memory storage capacity to store the sampled image. Furthermore, if a digital still camera is required to capture 1 frame/sec, a data transmission rate or bandwidth of approximately 2.5 Mbits/sec is needed for a color image. Depending on the digital processing application, the memory storage and bandwidth requirements can be even higher than 307 Kbytes and 2.5 Mbits/sec, respectively. Such large memories and bandwidths are either impractical or may require high capacity memory devices and very high speed circuit elements with parallel processing which can make the digital processing system complicated and very costly.
Thus, the large memory storage and channel capacity requirements for digital image transmission and storage make it desirable to reduce the amount of digital data from A/D converter 120. By reducing the amount of data, the transmission bandwidth and/or the memory storage requirements are reduced. A well-known technique is to utilize a digital signal processing (DSP) and image compression circuit 130 to reduce the amount of digital data while storing enough data in memory to maintain a desired quality or resolution of the image. Image compression circuit 130 can use common techniques, such as JPEG or run-length coding, to take advantage of the fact that certain pixel values tend to be highly redundant, such as with neighboring individual pixels or neighboring frames. Image compression techniques, e.g., having compression ratios ranging from 4:1 to 20:1, can significantly reduce the number of bits required to represent images by removing these redundancies while still maintaining an acceptable quality of the reconstructed image.
After image compression, the reduced digital data can be stored in a digital memory device 140, such as a memory card having a Flash EEPROM. Read and interface circuits 150 then access the desired image data from memory device 140 and transmits the data to desired digital destinations, such as a digital display unit 160, a digital printer 170, a hard disk drive for a PC 180, or another digital memory 190. For example, with a digital still camera, elements 110 to 150 are typically embedded within the digital camera so that the digital memory 190 can be a memory card which is removable from the camera and available for subsequent processing of the stored image data.
Thus, with image compression, smaller memories and lower transmission rates are possible. However, by using DSP and image compression, some information is lost, which can adversely affect picture quality. DSP and image compression also require substantial amounts of time for performing complex and lengthy computations, which can adversely increase the time delay between successive picture frames. A fast burst-rate, which allows pictures to be taken in rapid succession, is an important consideration for high-speed professional cameras. Furthermore, power usage and required silicon area is increased, which can increase the cost of the digital image processing system, and make such systems undesirable for applications targeting the typical consumer market.
For example, digital still cameras currently available for the average consumer have typical resolutions less than 500K pixels/frame, which result in much poorer picture quality than conventional film. While some digital still cameras have resolutions greater than 1M pixels/frame, these high quality cameras are generally too costly for the typical consumer. Therefore, a need exists for a low-cost digital camera capable of high resolution pictures which can replace conventional film cameras. The high cost of today""s digital still cameras is due primarily to higher internal memory storage and digital signal processing and image compression requirements, resulting in larger and more complex circuit elements. Accordingly, a simple, low-cost and high quality digital imaging system is desired which overcomes the problems discussed above with the conventional systems.
In accordance with an aspect of the present invention, a digital imaging system has a non-volatile analog/multi-level memory to store analog/multi-level data directly from an image sensor or voice data from a microphone. The data from the analog memory can then be directly transmitted to analog devices, such as an analog display, analog printer, or external analog memory, or to an analog-to-digital (A/D) converter for conversion to digital data. Once the image data is converted to a digital format, the data can be processed for any number of desired digital destinations, such as downloading to a hard disc drive (HDD) for permanent storage or editing by photo-enhancement software, transmitting through the Internet, or writing into removable memory cards (e.g., Sandisk""s Compact Flash or Intel""s Miniature Card).
By storing analog data instead of digital data, there is no longer the problem of storing large amounts of digital data, which eliminates the need for digital signal processing (DSP) and image compression. Without image compression, no information is lost, resulting in an imaging system providing higher quality images. Further, without the need for DSP and image compression circuits, the imaging system is smaller, simpler, less expensive, and requires less power.
Different types of analog memories can be used to store the analog data, with the type depending on the source of the image and/or the display. For example, for applications requiring a high bandwidth, such as high resolution digital still cameras which require high bandwidths in order to minimize the time required between consecutive exposures, data needs to be stored and read out at very high rates. Accordingly, the analog memory must be of a type capable of meeting these needs.
According to one embodiment, the analog memory contains multiple storage segments which are interleaved for sampling and storing values representing an analog signal and for transmitting the stored signal. Various embodiments of this type of analog memory are described in commonly-owned U.S. Pat. No. 5,680,341 to Wong et al., issued Oct. 21, 1997, entitled xe2x80x9cPipelined Record and Playback For Analog Non-Volatile Memoryxe2x80x9d, which is incorporated herein by reference in its entirety.
Each storage segment includes a sample-and-hold circuit and a write circuit coupled to a memory section associated with that segment and is capable of write operations that overlap write operations of other storage segments. Each storage segment can also include a read circuit and a sample-and-hold circuit coupled to an associated memory section and is capable of read operations that overlap read operations of other storage segments. Access to the storage segments are interleaved and operate sequentially during write or read, and the number of storage segments is selected according to a desired sampling frequency or bandwidth. Alternatively, both read and write operations of the storage segments can be carried out simultaneously and in parallel using multiple parallel pipelines, with each pipeline having at least one sample-and-hold circuit and at least one memory section. Analog memories of these types can accommodate the very high bandwidths required for certain high-end applications, such as digital still cameras and high fidelity music.
The analog signals stored in the analog memory can then be used as needed. For example, the analog signal can be directly read out to an analog display or analog printer, or the analog signal can be read out to an A/D converter for conversion to digital data. The resulting digital data can then be transmitted to desired digital destinations, such as an NTSC/PAL encoder for use with a television monitor or removable memory cards, or downloaded to a hard disk drive for photo-enhancement editing or later use by digital printers, displays, or transmission through the Internet.
Since the image is stored in the form of an analog signal and the analog signal is converted to digital data only when needed, the need for storing the digital imaging data is eliminated. As a result, image compression, which causes information to be lost, can also be eliminated. Without large digital memories and digital signal processing and image compression circuits, the cost of the digital imaging system can be greatly reduced while greatly improving the quality of the pictures.